As this kind of techniques conventionally known are a charged particle generator for generating charged particles (e.g. ions or electrons) from a charged particle source by means of a raw material gas and a high voltage, and an accelerator comprising the charged particle generator and an acceleration tube for accelerating charged particles, emitted from the generator, by application of an acceleration voltage. Above all known is a multi-stage acceleration type accelerator (e.g. see Patent Document 1) in which a plurality of acceleration electrodes are provided in a direction of a shaft center at predetermined spacings on the inner peripheral face of a cylindrical acceleration tube body, the adjacent acceleration electrodes are connected by a voltage dividing resistor, and a voltage obtained by dividing an acceleration voltage applied between the acceleration electrodes at each end is applied between the adjacent acceleration electrodes, to accelerate at multiple stages charged particles emitted from the charged particle generator.
In such an accelerator, high voltage resistant insulation needs to be performed for preventing discharge from the charged particle source to the outside of the charged particle generator and discharge over the surface of the voltage dividing resistor, and hence the charged particle source and the acceleration tube are typically sealed by a voltage resistant insulating gas such as SF6.
Patent Document 1: Japanese Patent Laid-Open No. H11-25900